Conventional optical measuring systems are used for detecting certain components in fluids or gases. Individual substances in said media or a substance concentration of these media can be ascertained with the aid of a spectroscopic measurement, such measurements requiring at least one optical detector and at least one optical radiation source. Since these components are usually not resistant to all fluid media to be tested, they must be protected.
Generally, the optical fluid sensors are calibrated following the final production step in order to achieve the required accuracy despite the tolerances of the individual components and the optical path. To do so, for example, the medium or fluid to be measured is introduced in a certain concentration, and a correction value or a correction function is calculated subsequently. Depending on the sensor type, this requires one or several calibration point(s). This constitutes a disadvantage in mass production, especially in the case of fluid sensors, since it is labor- and cost-intensive, and possibly even dangerous if flammable fluids are involved.
Optical fluid measuring systems may be configured for the following measuring principles:                Absorption-measurement methods, in which the fluid to be measured is situated between the radiation source and the detector, a distance between the radiation source and the detector being a function of the application requirements;        Reflection measurement methods, in which the fluid is characterized on the basis of reflection characteristics;        ATR measurement methods (attenuated total reflection) represent a measuring technology of infrared spectroscopy for checking the surface of opaque substances, such as lacquer layers or polymer foils, and also fluid samples such as solvent mixtures. An intensity of reflected light is measured in the process, thereby allowing conclusions with regard to the absorbing fluid.        
All enumerated measuring principles require a calibration of the fluid sensors independently of the application.